Aromatic polyamide film is being investigated in various applications to utilise its outstanding heat resistance and mechanical properties. In particular, para-oriented aromatic polyamide is superior to other polymers in its mechanical properties such as stiffness and strength, etc, so it is extremely useful in producing thinner film, and applications such as printer ribbons, magnetic tape and capacitors, etc, are being considered.
In recent years, along with the advance of digital recording technology and developments in computer external memory, etc, demands have increased for film suitable for thinner, higher recording density and highly durable magnetic recording media. That is to say, as the magnetic layers for realising high output, great progress has been seen in the formation of ultra-thin coated type magnetic layers and in metal evaporated type magnetic layers where the magnetic layer is formed directly on the film, but the higher the performance of the magnetic layer the greater the demand for the base film to attain high levels in terms of smoothness, running properties and lack of defects. Again, as the performance of the magnetic layer is further raised, so the film processing conditions involve ever higher temperatures and speeds, etc, and the burden applied to the base film becomes ever larger.
However, there are the following problems in the case where aromatic polyamide film is used. Firstly, if the surface is made smoother in order to raise the output characteristics, then, due to friction with the head, the magnetic layer tends to separate away and clogging of the head may occur due to the separated magnetic powder. Moreover, if surface roughening is carried out to improve the durability in this respect, then the output characteristics demanded of a high performance magnetic recording medium are no longer obtained and the head may be damaged by the surface projections. Secondly, if the surface is roughened to confer good running properties, transfer of such roughness to the magnetic layer may arise and loss of data by the magnetic recording medium may occur.
As examples where aromatic polyamide film has been used in magnetic recording media applications hitherto, there is the example where the height of minute projections at the surface and the diameter and number of the projections are prescribed (Japanese Unexamined Patent Publication No. 60-127523), and the example where the height of minute projections at the surface, the average diameter, the flatness of the projections, the diameter and number of the projections, and the ovalness, roundness of the projections are prescribed (Japanese Unexamined Patent Publication No. 61-246919), etc, but with these it may not be possible to achieve the extremely high levels of both output characteristics and durability to meet the latest advances in magnetic recording media.
Again, in Japanese Unexamined Patent Publication No. 3-113819, there is disclosed a magnetic recording medium in which an undercoat layer is provided on a non-magnetic support and there are present in this undercoat layer aggregated clumps, but coarse projections are readily formed by aggregated clumps and in some cases dropout becomes considerable. Moreover, by employing an undercoating stage, the productivity may be reduced.
Further, in Japanese Unexamined Patent Publication No. 8-77554, there is disclosed a magnetic recording medium where a ground layer is provided on a non-magnetic substrate and the height and number of projections on this ground layer are prescribed, but the regulation of the projection height is insufficient to achieve both the extremely high levels of output characteristics and durability to meet the latest advances in magnetic recording materials and, moreover, by employing a ground layer forming stage the productivity may be reduced.
As a result of intensive investigation into the causes of such problems, the present inventors have found that the height distribution of the projections at the surface of the aromatic polyamide film plays a considerable part, and they have discovered that by the optimisation of the projection height distribution it is possible to establish extremely high levels of both output characteristics and durability. It is on this discovery that the present invention is based.